Discounting total amount due numeric field

I am working on a registration form and need to apply a discount if two particular checkboxes are selected. 

Checkbox #1 – Early registration = $1595

Checkbox #2 – Standard registration = $1795

Checkbox #3 – Special rate = $495

Checkbox #4 – Optional workshop = $395

Numeric Field – Total Payment Due

If the user selects checkbox #1 (early) or #2 (standard), they have to pay the $395 for the optional workshop.

     Total box =

     sum (EarlyReg, StandardReg, SpecReg, WorkshopReg) <-- THIS WORKS

If the user selects BOTH checkbox #3 (special registration rate) AND checkbox #4 (optional workshop), they do not pay for the workshop.

So instead of $890, they will pay $495.

     Total box =

     ?????

I welcome any ideas you may have and would love to hear if you think I'm going at this in the wrong direction.

I am new to this...would appreciate a step-by-step with syntax. Thanks!